1. Field
The present disclosure relates to a vanadia-titania catalyst for removing nitrogen oxides and a method for preparing the same. More particularly, the present disclosure relates to a vanadia-titania catalyst obtained by forming titania carriers (particles) via chemical vapor condensation, and supporting vanadia onto the titania carriers (particles) through impregnation and calcining, and thus having a high specific surface area, uniform and fine nanoparticle size and excellent vanadia dispersibility, and particularly exhibiting excellent nitrogen removal efficiency at a low temperature range of 200-250° C., as well as to a method for preparing the same.
2. Description of the Related Art
In general, a great amount of nitrogen oxides are discharged from power plants or waste incineration plants, and function as a main cause of air pollution. Such nitrogen oxides comprise N2O, NO, N2O3, NO2, N2O5, NO3, or the like. Particularly, NO and NO2 are regarded as typical nitrogen oxides discharged from power plants or incineration plants.
As methods for reducing nitrogen oxides, there are methods of optimizing combustion conditions to reduce generation of nitrogen oxides per se during combustion and methods of posttreatment comprising subjecting nitrogen oxides generated after combustion via selective non-catalytic reduction (SNCR) or selective catalytic reduction (SCR).
More particularly, SCR comprises introducing ammonia or urea to the front end of a catalyst together with exhaust gas to reduce and remove the nitrogen oxides in the exhaust gas. During the reduction, the nitrogen oxides are converted into water and nitrogen and then removed while they react with ammonia and the catalyst, as shown in the following reaction formulae:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2O
Such an SCR method provides excellent nitrogen oxide removal efficiency, and thus has been used commercially in most incineration plants or power plants in practice. The catalyst mostly uses titania (TiO2) or its acidity-modified form, comprising TiO—SiO2 or TiO2—ZrO2—SiO2, as a carrier (support). As an active metal, a composite oxide comprising a combination of V, W, Mo, Sn, Ce, Mn, Cr and a noble metal has been used. With respect to the quality of a catalyst for removing nitrogen oxides, not only the activity of an active metal but also the quality of a catalyst carrier functions as an important factor determining the overall quality of the catalyst. In general, most catalysts for removing nitrogen oxides use TiO2 as a carrier (support). TiO2 is significantly more excellent than Al2O3 or zeolite in terms of electron transport. Therefore, TiO2 provides a desired effect not only as a catalyst carrier but also in a removal reaction.
For example, Korean Laid-Open Patent Publication Nos. 10-2005-0031037 and 10-2011-0034400 disclose a catalyst for removing nitrogen oxides using TiO2 as a carrier, and a method for removal of nitrogen oxides (denitrogenation) using the same.
Carriers of a catalyst for removing nitrogen oxides have also been prepared by a wet process, such as co-precipitation or impregnation process. However, such a process requires a relatively large number of operations, comprising dissolution, evaporation, drying, pulverization and calcining. Thus, it takes a long time of several days or more to prepare a carrier for catalyst by such a process. In addition, the prepared carrier has a relatively large primary particle size, shows low dispersibility when supporting a noble metal or transition metal thereon, and provides a small specific surface area. As a result, the final catalyst provides low catalytic activity as well as a small specific surface area, and particularly shows low activity at a low temperature range, thereby making it difficult to provide high nitrogen oxide removal efficiency.
In addition, it is known that the SCR method shows the highest nitrogen oxide removal efficiency at a temperature of 300-400° C. The temperature of exhaust as introduced to an SCR system, i.e. to a catalytic reactor operated in a power plant or incineration plant is about 200° C., which is lower than the active temperature of a catalyst. Thus, according to the related art, an exhaust gas heat exchanger of supplementary fuel has been used to increase the temperature of exhaust gas, and then the warmed exhaust gas has been introduced to carry out reaction in an SCR system. However, in this case, addition cost is required for installation and maintenance of a heat exchanger, and energy cost is increased due to high oil price, thereby increasing the manufacture cost of a catalyst. Therefore, there is a need for a technology of preparing a catalyst capable of removing nitrogen oxides sufficiently at a low temperature range less than 300° C., particularly between 200° C. and 250° C.